The well of concepts for bottom-up blocks - that is, those designed mechanics-first with flavor conforming to the actual gameplay themes that are supposed to be advanced - runs deep, but is definitely finite. After all, there are only so many parts of a game of Magic that can be interacted with in enough ways to build a whole block around. This is why block themes have repeated over the years; fortunately, many players are fond of bottom-up blocks simply because their favorite kinds of cards or decks get new variations.
Still, it came as some surprise when loki7289 asked Mark Rosewater on his Tumblr blog, "a graveyard matters block or set on the storm scale" and got a definitive response:
That’s a 1. It’s a theme we keep coming back to as it’s a rich design vein.
There are a couple of presumptive routes for a graveyard-matters block already built into previous sets: One is the two graveyard-based futureshifted mechanics from Future Sight, gravestorm and delve - though those may be unlikely to ever reappear because of abuseability concerns - and the other is the anticipated Innistrad sequel block, which will presumably bring the Gothic-esque world under threat once again because the archangel goddess Avacyn loses power or betrays her people.